The present invention relates to boiled rice foods capable of being distributed at low temperatures. Specifically, the present invention relates to boiled rice foods having a texture which does not deteriorate following distribution and storage at low temperatures, and which are free from hygienic problems associated with warming or thawing at room temperature.
More specifically, the present invention relates to boiled rice foods having a texture which does not deteriorate when warmed or thawed following chilled or frozen distribution, and which are free from any hygienic problems associated with warming or thawing at room temperature. In the present invention, "chilled distribution" refers to distribution and storage in a temperature below about 5.degree. C. and above a freezing point; "frozen distribution" refers to distribution and storage in a temperature range which enables freezing; while "de-chilling" refers to warming in a temperature below about 10.degree. C. and above a freezing point.
Cooked boiled rice hardens over time. This phenomenon is also called deterioration of boiled rice. The degree of deterioration increases at lower temperatures. Such deterioration results from changes in intrinsic starch and it has been suggested that the mechanism involves recrystallization of starch which has been gelatinized and swollen under the influence of water and heat. In particular, the taste, texture and flavor of boiled rice all significantly deteriorate after distribution and storage at a temperatures sufficiently low to cause chilling or freezing.
In order to prevent the deterioration of boiled rice, attempts have been made which utilize methods such as increasing the amount of water added for cooking or adding enzymes such as .alpha.-amylase, .beta.-amylase, glucoamylase and protease, starch such as pregelatinized starch and chemically modified starch, polysaccharides such as Duran gum and hyaluronic acid, sugars such as anhydrosugar, dextrin, glucose, fructose and cyclodextrin, sugar alcohols such as trehalose, maltitol, sorbitol and lactitol, or fatty acid esters such as sucrose fatty acid ester, polyglycerin fatty acid ester and glycerin fatty acid ester, and emulsifiers such as organic acid esters of monoglycerides. However, none of these methods achieve any significant improvement in taste or texture.
When boiled rice foods containing these additives are chilled or frozen for distribution, various methods are available to effect de-chilling or thawing, such as allowing the food to stand at room temperature for a period of time, warming at a relatively low temperature (de-chilling), or thawing by heating in a microwave oven. However, if foods are left to stand at room temperature for a period of time, a hygienic problem can arise. In addition, thawing by heating is not suitable for boiled rice foods such as sushi. If the microwave method is used for more suitable foods, hardening is likely to result, causing an undesirable taste, texture and flavor. The same problem exists with respect to de-chilling. When boiled rice foods produced in a conventional method are subjected to de-chilling, such foods will suffer a deterioration in taste and texture.
As a countermeasure against such deterioration, a method of adding trehalose or trehalose plus sugar alcohol (e.g. JP-A 8-168350 and JP-A 9-163943) has been proposed. This method is said to enable foods to be returned to an edible state without heating following chilled or frozen storage and distribution. However, no improvement in taste and texture can be achieved using de-chilling, thus such foods should be thawed at room temperature. Thus, the problem of hygiene is not solved thereby. Further, thawing at room temperature does not effect a satisfactory improvement in taste and texture.
The object of the present invention is to provide boiled rice foods the taste, texture and flavor of which do not deteriorate when returned to an edible state by hygienic de-chilling after being distributed in a chilled or frozen state.